A Budget Analysis of Bottom-Water Dissolved Oxygen in Chesapeake Bay

Abstract

A coupled hydrodynamic-dissolved oxygen model is developed to simulate the seasonal cycle of dissolved oxygen (DO) in Chesapeake Bay and investigate processes regulating summer hypoxia in the estuary. A budget analysis of DO in the bottom water reveals a balance between physical transport and biological consumption. In addition to the vertical diffusive flux, the longitudinal and vertical advective fluxes are important suppliers of DO to the bottom water. The longitudinal advective flux is affected not only by gravitational circulation but also by wind-driven currents. The vertical advective flux is affected by wind-driven lateral circulation and shows a strong dependence on wind speed and direction. Up-estuary winds weaken the landward bottom flow and generate a clockwise lateral circulation that exchanges DO between the deep channel and adjacent shoals, thereby reducing the longitudinal advective flux and increasing the vertical advective flux. In contrast, down-estuary winds amplify the longitudinal flux and reduce the vertical flux. During the summer, water column respiration contributes to about 74 % of the total biological consumption and sediment oxygen demand accounts for about 26 %. Sensitivity analysis model runs are conducted to analyze how changing river flows and winds affect the hypoxia prediction and oxygen budget balance. Due to the compensating changes in longitudinal and vertical fluxes, the hypoxic volume is relatively insensitive to changes in the river flow. In contrast, the timing and size of hypoxic volume changes with wind speed. Sensitivity analysis also shows that plankton oxygen production, water column respiration, and sediment oxygen demand all affect the hypoxia prediction and bottom oxygen balance.